This post was originally stuff from another thread, and now that there's this subforum, it was thought better to put it in its own thread.

I made a video effects box which perhaps only slightly qualifies as doing synthesis, and it deals with sync in a rather simple manner. No sync generator.

Basically, it splits a composite video signal, slices it up using an LM339 comparator which clocks a 4040 divider (that's where the CMP - DIV of the title comes from), then adds a few of the divisions back to the original signal. Since the original signal is still there, the sync is preserved. Limited, but some fun stuff for such a dumb circuit.

My friend who actually came up with the circuit had originally conceived it for audio, but then we wondered what it might do with video. We each took it in slightly different directions, but the core idea remains.

Warning: Use at your own risk. I cannot guarantee that this won't break equipment this is connected to. In certain cases, this can add a DC offset to the output signal, so it ends up between 1V and 2V. It is beyond my electronics knowledge to know if this is bad or not.

James.

P.S. I used this circuit to produce my avatar-self-portrait... those wavy lines are classic CMP-DIV effects.

Edit: Changed my website URL.

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Last edited by RingMad on Thu Sep 25, 2014 6:42 am; edited 1 time in total

I have no way to test it, but I don't see any reason why it would not work on PAL. Since the input and output is composite video, then that should be relative to whichever system it's used on.
It's pretty easy to build, so you could try it! If you do, please report your findings here.

well, I got the components I didn't have and I build it and... it doesn't work

Sorry to hear that. I know of no-one else who built this. I'll examine your breadboard after work, but could you tell me what happens? Do you see anything at all?
With the "raw level" pot at 0 ohms or bypassed, even with the circuit powered off, you should see your original input signal, since if you look at the circuit, it would go straight through.

the pots coming out from the 4040 does affect a little the output signal (when the "raw" allows it to pass through), but there is close to no change, and it seems to be more like a "threshold" o desaturation kind of effect rather than another thing. no wavy lines here

OK, I checked against your breadboard image and it looks like you connected the circuit correctly.

Now, when testing, keep the raw level at max, i.e. 0 ohms, so as to not choke the sync. If you choke it too much, then the image will loose sync. (It's just that I found one can get interesting effects by playing with this (very gently), once one has found the right threshold level).

I guess I should have explained better about the threshold, because it's the most important and difficult part. To start, leave the 1K fine tuning in the middle of its range. Now, slowly sweep the coarse threshold pot, watching your screen carefully. You might easily pass by the right spot. It might just be a quick flicker on your screen. If you see something like that, make very small movements back and forth near to where it flickered and try to find the sweet spot, then use the fine tuning pot to better pinpoint it.

I looked at the signal diagrams for PAL and it's quite similar to NTSC in its overall range, but perhaps with a small offset. I don't really know how that all works, but perhaps for PAL, a larger pot is needed in order to find the sweet spot. Try 100K instead of 50K... but turn it even slower, because the higher the value of the pot, the less precision there is.

Also note that different input signals may have different threshold settings. e.g. from a VCR or a camera, or a DVD player. One always has to adjust it to find the good level.

I'm sorry it didn't work right away for you... right now you are the PAL pioneer... exploring where no-one has!

Glad you got it working, Javier! So it is confirmed that this circuit does indeed work for PAL as well as NTSC. Good to know.

jjplano wrote:

it's possible that you can replace the 5k pots coming out of the 4040 for 1k or even less value?

I note image distortion/change in the really lower end of the turn, really close to 0 ohms

For the different kinds of video sources I tried (VCR, camera), I found that 5K was a good range for me. If you feel that 1K is better for you, go ahead and use that.

Another tricky thing is that the setting of one pot can affect the settings of the others. For example, if one pot is all the way "up", then maybe another pot will have little effect, but if the pot is halfway, then the other pot might begin to have an effect. I don't really understand it... I think it has to do with the resulting overall level vs the sync. Not enough and you lose it, too much and maybe it distorts. It's a dumb circuit, but interesting things can be found... one has to explore.

Another tricky thing is that the setting of one pot can affect the settings of the others. For example, if one pot is all the way "up", then maybe another pot will have little effect, but if the pot is halfway, then the other pot might begin to have an effect. I don't really understand it... I think it has to do with the resulting overall level vs the sync.

May I just chime in here and say it may have more to do with your mixing methods for the 4040 outputs and raw signal.
I can see one or two scenarios where things could be shorted out by the pots.
Give me a shout if you want my modification ideas._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

This will definately need testing; if you would be so kind Mr jjplano, that would be helpful, but no dire rush or anything
This new arrangement should allow greater functionality and stop the controls interfering with each other.

I would advise a non-inverting amplifier at the end with a gain of about 4 (I think) to bring things back up to the level they were originally. However that will need testing/experimenting with, I don't know what kind of voltage the video input is expecting.

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_________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Thanks for the mods/advice JingleJoe... if I have a moment sometime to try them out, I will.

JingleJoe wrote:

I don't know what kind of voltage the video input is expecting.

Although technically a video signal can be any amplitude, I've measured NTSC composite video signals as 1 V p-p about. There's a special relative unit used to accommodate that, but in this case it would work out to ranging from -286 mV to +714 mV.

I've measured the signal coming out of my Vidiffektor box and it goes from about +1 V to +2 V, so still maintains 1 V p-p but with a 1 V DC offset up.

I've tried to figure out what the original circuit actually does and my best guess is it works as a phase lag circuit by messing with the output impedance, and it can also impose a small level shift depending on the pot configuration on the 4040 and the 339 output level.

Joe's modified circuit cannot work as the video signal source impedance is 100k driving a load of 75 ohms, so the video out will be 750 uV at most. The variable 1k max pot in the original circuit is a huge series resistance for a video signal to see and I'd guess the circuit works best with it set to 250 ohms or less. RingMad mentioned the comparator threshold setting is critical and I'd agree (without having built the thing of course). The threshold must be set in the video signal and probably above the colour burst level where it could screw up the black level clamping which, depending on your tv, could result in no output.

Subsequently I would advise using my circuit with a voltage follower on the end. I thought the high resistors might cause problems however I didn't know what sort of input impedance we were talking about, thankyou for clearing that up Brock
Any problems with impedance in my altered circuit could be overcome by adding a voltage follower to the output, a simple op amp buffer. It may need a resistor on the output (of about 250 ohms as brock suggests) However one may also need to check the max power/current output on the datasheet of the op amp used but most typical high speed op amps should be fine._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Just thought I'd post my build of James' CMP-DIV circuit. Here I've included the circuit in a box with two 'dirty video mixers' (just one at the moment) to give a processor that can be used for blending video sources together on the fly, as well as adding some colour distortion and distinctive CMP-DIV effects.

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